This invention relates generally to DC-AC power converters and more specifically to DC-AC converters which serve as drivers of AC-DC converters.
There are two popular approaches to the design of DC-AC converters, both of which convert a DC source voltage into an AC output by switching power semiconductor switches on and off at a high frequency. These approaches provide the means for achieving small, light-weight, and highly-efficient converter drivers.
One approach is to generate a stream of pulses and to control the output voltage by controlling the width of the pulses. This pulse-width modulation (PWM) approach results in square-wave voltage waveforms across the switches and switching losses that increase with switching frequency. The switching losses tend to be high, and the electromagnetic interference (EMI) that accompanies the process is also high and difficult to control by filtering.
A second approach to DC-AC conversion is to add capacitor-inductor resonating elements to the PWM configurations in order to obtain sinusoidal voltage and/or current waveforms. These resonant converters have lower switching losses, thereby permitting operation at higher switching frequencies. The EMI generated by resonant converters is lower, and the higher switching frequencies result in reductions in size, weight, and cost. However, resonant switching means that the semiconductor switches are subjected to greater stress, and switches designed for greater stress also have larger "on" resistances which tends to increase switching losses. Many resonant converters operate with varying switching frequencies which means the generated EMI is harder to predict and control.
There is a need for DC-AC converters that combine the simplicity of PWM converters with the performance advantages of resonant converters.